An electrically-driven vehicle drives an electric motor in accordance with a range selected by a shift manipulation which is done by a driver to produce a running mode of the vehicle. By transmitting this power from the electric motor to road-wheels, an electric running can be realized.
When the electrically-driven vehicle is made to run at very low speed (is made to creep) by creep torque with a forward-or-reverse-running range selected as in the case of automatic-transmission vehicle; the electric motor is driven and controlled to output the minute creep torque from the electric motor, and this creep torque is transmitted to the road-wheels to enable the creep running of the electrically-driven vehicle.
For example, Patent Literature 1 proposes a creep-running control technique for an electrically-driven vehicle. In this technique, in addition to enabling the creep running of the electrically-driven vehicle as mentioned above, a creeping-cut which reduces the creep torque of the electric motor (down to zero) is performed when a predetermined creeping-cut permitting condition is satisfied. This predetermined condition is that a detection value of vehicle speed is lower than a set value and also a braking force is larger than or equal to a set value, for example, when the vehicle is in a stopped state with no intention to start moving the vehicle. Such a creeping-cut is performed because the creep running is not conducted immediately and also because a power consumption can be suppressed.
However, in the creeping-cut control apparatus for electrically-driven vehicle as disclosed in Patent Literature 1, the creeping-cut which reduces the creep torque of the electric motor (down to zero) is unconditionally performed when the predetermined creeping-cut permitting condition is satisfied, for example, when the vehicle is in a stopped state with no intention to start moving the vehicle. Therefore, the following problem is caused.
For example, a case will now be discussed where the electrically-driven vehicle located on an upwardly-sloping road moves downwardly in a reverse-running direction because a stopped state of the vehicle cannot be maintained by the creep torque when a forward-running range (DRIVE-mode) is being selected. In this case, in order to prevent the downward movement in the reverse direction, the driver brings the vehicle speed close to 0 by depressing a brake pedal and thereby applying a braking to the vehicle or by increasing the depression of the brake pedal and thereby enlarging a braking force for the vehicle.
During this downward movement in the reverse direction, (an absolute value of) the vehicle speed is not near 0. Hence, the creeping-cut permitting condition is not satisfied regardless of a state of the braking, so that the creep torque is being outputted from the electric motor. Then, when (the absolute value of) the vehicle speed becomes near 0 by a braking of brake-pedal manipulation for preventing the reverse-directional downward movement of the vehicle, the creeping-cut permitting condition is satisfied, so that the creep toque which is outputted from the electric motor is brought to 0 by the creeping-cut.
Before the creep torque is brought to 0 by such a creeping-cut, the driver is adjusting his depressing force of the brake pedal to balance with a gradient resistance of the upwardly-sloping road in consideration of the creep torque. Therefore, when the creep torque is brought to 0 by the creeping-cut, the driver feels a torque reduction unrelated to his own driving manipulations, i.e., an unintended torque reduction. Hence, a problem is caused that the driver has a feeling of strangeness.
This problem is caused similarly also when the electrically-driven vehicle moves downwardly in the forward direction under a reverse-running range (REVERSE-mode), i.e., when even a stopped state of the vehicle cannot be maintained by the creep torque although the driver is trying to run the vehicle in the reverse direction on a sloping road by the creep torque.
That is, in the case that the electrically-driven vehicle moves in a direction opposite to the running direction of the selected running range against the creep torque outputted according to the selected running range, the above-mentioned problem is caused when (an absolute value of) the vehicle speed is reduced to a value near 0 by a braking performed to prevent this opposite-directional movement of the vehicle and thereby the creeping-cut permitting condition is satisfied.